1. Field of the Invention
This invention relates to electrolytic baths useful for depositing metals on or removing metals from substrates and to electrolytic methods using such baths.
2. Description of the Art
A variety of electrolytic methods are widely employed to deposit metals on or remove metals from conductive substrates for a variety of purposes. Electrolytic plating, including impressed current and sacrificial (immersion) plating, is used to produce decorative coatings of gold, silver, copper, chromium, nickel, or other metals on a variety of substrates. Plating is also employed to improve the corrosion resistance of corrosive substrates by depositing thin surface films of corrosion resistant metals such as zinc, tin, chromium, nickel and others. Wear resistant and friction modifying coatings of nickel, chromium, titanium, and other metals and their alloys are used to improve the wear resistance of bearing surfaces. Electroplating is also widely employed in the electronics industry to improve or modify the electrical properties of substrates such as contacts, printed circuits, electrical conductors, and other electrical apparatus in which specific surface or surface-to-substrate conductive properties are desired. Dissimilar metals are often electroplated onto metal surfaces to improve soldering characteristics or to facilitate subsequent coating by painting or application of other adhering films such as plastics, adhesives, rubber, etc.
Electroforming is a special application of electroplating in which usually thicker deposits are formed on a substrate which, typically, is later removed from the electroformed deposit. Such methods enable reproduction of fine detail in the substrate and are often used for the manufacture of templates for reproducing articles having fine surface features such as phonograph records
Electrowinning processes represent another application of electroplating and are employed for the recovery of valuable metals from ores and scrap. Typically, the desired metal is first converted to an acid-soluble form such as the oxide by calcining the ore or scrap, after which the oxide is dissolved in an acid such as sulfuric, nitric, etc., and recovered from solution by electroplating.
Electrolytic metal removal is used for electrolytic machining, polishing, roughening and anodizing. Such processes involve placing the metal to be treated at the anode rather than the cathode and removing a portion of the metal from the article. Electrolytic machining usually involves masking a portion of the article's surface to expose only those portions to be removed, and it enables the rapid accurate machining of complex articles such as turbine blades and other machine parts. Electropolishing usually involves removal of metal from the cathode surface only at the high points of irregularities with little or no dissolution of metal at low points or valleys and is used to produce smooth lustrous finishes on metals such as stainless steel, carbon steel, brass, aluminum, silver, nickel, copper, zinc, chromium, and gold. In contrast to electropolishing, electrolytic roughening involves anodic treatment of a metal surface using special types of electric current such as an alternating current in which the current strength has an anode amplitude greater than the cathode amplitude. Such treatment unevenly removes metal from the anode surface and introduces or amplifies surface roughness on a microscopic scale. Illustrative electrolytic roughening processes are disclosed in U.S. Pat. No. 4,087,341 and are usually employed to improve surface adhesion and/or susceptibility to subsequent treatment such as anodizing. The latter treatment--anodizing--involves anodic metal removal combined with oxidation and is typically employed to provide a metal oxide coating to improve corrosion resistance, surface texture, adhesion to films or laminates, or modify electrical properties. Anodizing is typically carried out in aqueous acidic electrolytes which can contain various proportions of acids such as sulfuric, phosphoric, chromic, amidosulfonic, sulfosuccinic, sulfosalicylic acids and mixtures of these as disclosed in U.S. Pat. Nos. 4,482,444, 4,211,619, 4,049,504, and 4,229,266.
Many of the present electrolytic methods for depositing or removing metals involve the use of strong acid electrolytes, and others could be effected in the presence of strong acid electrolytes and/or higher acid concentrations could be employed if several of the problems associated with acid electrolytes could be overcome. Strong acids are ideal solvents for plating metal ions and compounds and for metal ions and compounds removed during anodic treatment. They are also highly conductive and therefore introduce little electrolyte resistance to current flow. However, current densities and cathode voltages required to effect the necessary cathodic reduction and/or anodic oxidation often exceed the hydrogen over-voltage potential at the cathode and/or the oxygen over-voltage potential at the anode, either of which can introduce significant complications into the process, detract from product quality, or, in severe cases, render electrolytic methods totally impractical. Cathode potentials which exceed the hydrogen over-voltage potential result in hydrogen evolution, irregular and/or non-adherent metal deposition, hydrogen embrittlement, especially with high strength steels, and amplify pre-existing differential potentials across the cathode surface. Cathode surface potential variations promote the problems referred to above, and accentuation of such potential difference exacerbates those problems. Hydrogen evolution and other factors associated with localized or general cathode voltages in excess of the hydrogen over-voltage potential also promote surface pitting, waste electrical energy and can create an explosion hazard.
Localized or general anodic voltages in excess of the oxygen over-voltage potential cause oxygen evolution at the anode, create local distortions in current flow, impair product quality, waste electrical energy, and can create a hazardous, explosive atmosphere. Another disadvantage associated with the use of strong acids in electrolytic processing involves the corrosivity of such acids for the treated substrate or coating.
Thus, the use of strong acids, particularly in high concentrations, is often avoided since such acids generally detract from the quality of the finished product and complicate process control. In particular, strong acids, especially when employed at high concentrations, diminish the homogeneity, brightness, and dimensional conformity of treated surfaces, reduce the tenacity of plated metals, waste energy in the production of hydrogen and/or oxygen, and, at best, limit the current density range and electrode voltages which can be employed.
A variety of steps have been employed to minimize or eliminate the negative effects of strong acid electrolytes including avoidance of strong acid electrolytes altogether, minimizing acid concentration, the addition of corrosion inhibitors and/or sophisticated leveling and/or brightening agents, limiting current density at the cathode and/or anode, or avoiding electrolytic treatment involving combinations of electrolytic baths and substrates when their use would be unacceptably expensive or impractical. For instance, sulfuric acid, one of the least expensive yet strongest mineral acids, is considered, by some authorities, as too corrosive to be used as an electrolyte for electrolytic machining as discussed in the Kirk Othmer Encyclopedia of Chemical Technology, Third Edition, Volume 8, John Wiley & Sons, pp. 751-763 (1979). Other investigators have sought to mitigate one or more of the disadvantages associated with strong acid electrolytes by the use of additives including surfactants, leveling and brightening agents, and corrosion inhibitors. Thus, urea is disclosed as a corrosion inhibitor in U.S. Pat. No. 4,482,444, for the electrolytic treatment of aluminum in a dilute sulfuric acid electrolyte. U.S. Pat. No. 4,488,942 discloses the use of thiourea to improve the crystal structure of zinc plate obtained in an acidic electroplating solution and to broaden the permissible current density range. Thiourea is also disclosed in combination with organic brighteners to facilitate cadmium plating from acidic baths in U.S. Pat. No. 4,293,391 and as a copper complexing agent for the immersion plating of tin on copper substrates in the presence of acidic electrolytes in the Kirk Othmer Encyclopedia of Chemical Technology, supra, page 859. Zinc electroplating baths involving acid-containing electrolytes, certain tertiary amine surfactants, and brightening agents including aldehydes, ketones, carboxylic acids, and certain pyridine compounds, are discussed in U.S. Pat. No. 4,384,930. Various leveling agents have also been disclosed such as condensation products of thiourea and aliphatic aldehydes (U.S Pat. No. 3,101,305) and condensation products of certain epihalohydrins and certain nitrogen-containing compounds such as substituted pyridines (U.S. Pat. No. 4,038,161).
All of the remedial procedures developed thus far suffer from one or more disadvantages. They typically require the use of relatively dilute acid electrolytes, the elimination of strong acids altogether, or relatively low current densities and/or electrode surface potentials. They also require the use of relatively expensive, often short-lived, additives such as the brightening and leveling agents and/or surfactants, some of which are referred to above.